The invention relates to devices for transferring articles and, in particular, the invention pertains to an apparatus for moving sample holders past a series of operating stations.
Conventional transfer devices generally include endless belt or chain arrangements having pins or extensions for engaging the articles along an opening and transferring the articles about a predetermined path. The endless belt or chain is driven by a stepper motor which tops the article intermittently so that particular functions may be performed at the workstations.
A problem encountered in the conventional transfer devices is the misalignment between the engaging elements of the endless belt or chain and the articles being transferred. For example, the articles may include a sample holder having a row of compartments for containing the sample tubes, so that the engaging portion of the belt engages the holder along an opening for moving the sample holder along the path of the chain. However, if the engaging element fails to engage the opening, the holder becomes misaligned, thereby requiring the transfer operation to be stopped until the holder is reset.
Misalignment occurs even more frequently when the sample holder is transferred by frictional engagement between a belt and the holder, i.e., without engaging elements disposed along the chain or belt to engage openings in the sample holders. Not surprisingly, the friction between the sample holder and the chain or belt in this situation is generally insufficient to transfer the sample holders with the requisite precision. Therefore, such transfer devices typically require resetting to ensure the samples are aligned with the appropriate workstation.
Thus, another problem with these stepper motor controlled devices is that they require precision position sensing devices, position feedback circuits, and an alignment correction function, which is usually microprocessor controlled, in order to correct for cumulative position errors.
Another problem with conventional transfer devices concerns the manner in which the sample holders are deposited into the transfer path which goes past the workstations. For example, in some conventional devices, a first belt drives the sample holders in a first direction toward a wall having a workstation placed therealong, while a second endless belt or chain with projections engages the sample holder for transfer against the wall in a second direction at a 90.degree. angle from the first direction, to move the sample holder past the workstation. Although the second endless belt may be driven by a stepper motor, thereby providing intermittent movement, the first belt is driven continuously so that sample holders are pushed up against each other in the areas where they are deposited by the first transfer belt to the second transfer belt. Such an arrangement fails to provide any timing for introducing sample holders to the workstation transfer and results in the unnecessary continuous driving of the sample holders against an end wall.
Another problem with the prior art is the failure to provide a timing and monitoring apparatus for coordinating operations at a plurality of workstations.